Voice services over Adaptive Multi-user on One Slot (VAMOS) allows for voice capacity enhancement in GSM/EDGE Radio Access Networks (GERAN).
In GSM, each mobile station is assigned to one of eight available time slots per TDMA frame. In VAMOS, the voice capacity in GERAN can be improved by multiplexing two users, i.e. two mobile stations, on the same time slot and the same carrier frequency; the multiplexing of two mobile stations on a common time slot may be done in downlink direction (i.e. from the base station to the mobile station) and in uplink direction (i.e. from the mobile station to the base station). When multiplexing two mobile stations on a common time slot, the communication with the first mobile station is performed via a first VAMOS subchannel, and the communication with the second mobile station is performed via a second VAMOS subchannel. The users form a VAMOS pair.
In GSM, Discontinuous Transmission (DTX) is used when a speech silent period is detected during a speech connection. DTX operation is mandatory for GSM mobile stations.
The performance of the active Traffic Channel (TCH) in VAMOS can be improved if the TCH of the other user in the same VAMOS pair is in silence (inactive TCH), i.e. if the VAMOS subchannel of the other user is in DTX mode. This is due to the reduced interference originating from the VAMOS subchannel in DTX mode. However, the related Slow Associated Control Channel (SACCH) does not obtain such a performance benefit because a SACCH slot is transmitted per multiframe, regardless whether the VAMOS subchannel is in DTX mode or not. Thus, the SACCH of the VAMOS subchannel in DTX mode interferes with the SACCH of the other VAMOS subchannel.